The art of manufacturing tennis balls has been well developed heretofore by such companies as Dunlop Rubber Company, Limited, of London, England as shown by its U.S. Pat. Nos. 4,151,029 and 4,248,658 and Great Britain Patent No. 436,100. The elastomeric core of a tennis ball is usually made of vulcanized rubber molded into a pair of hemispherical rubber cups which are then adhered together by a heat curable cement. The core is heat treated to bond the hemispherical rubber cups to one another before the fabric cover is applied.
Since tennis balls are internally pressurized to provide a desired bounce characteristic, the core halves are assembled in a pressurized environment so that a suitable internal pressure, on the order of 17 lbs. per square inch, is provided within the core during its initial manufacture. When the core is subjected to the adhesive heat curing step, the mass of air within the core tends to expand the core against the surrounding heat applying mold. It is thus necessary to allow the mold and core to cool sufficiently after a core curing steps so that the core will not blow apart under the elevated internal pressure caused by the heated mass of air contained therein. Furthermore, the pressurized core subjected to a second heating step when the fabric cover is bonded to the core. A method of making tennis balls as set forth above is fully described in applicant's U.S. Pat. No. 4,853,057.
To avoid pressurizing the core during manufacture, a one way air valve may also be provided in the core. Generally, during manufacture the core is only partially pressurized and the fabric cover placed therein. Air may be introduced through the one way valve through the fabric covering after manufacture, thereby reducing the chance of core damage caused by excessive pressure built up within the core during heating. Such a method of making a pressurized ball is disclosed in applicant's U.S. Pat. No. 4,765,853.
In each of the above patents, it is noted that the core is formed by cementing two core halves together. The cemented core halves are then subjected to a cement curing step to bond each half to each other. It would highly desirable to provide a seamless hollow core to eliminate this step.